Structure of the voltage-gated potassium channel K1.3: Insights into the inactivated conformation and binding to therapeutic leads.

The voltage-gated potassium channel K1.3 is an important therapeutic target for the treatment of autoimmune and neuroinflammatory diseases. The recent structures of K1.3, Shaker-IR (wild-type and inactivating W434F mutant) and an inactivating mutant of rat K1.2-K2.1 paddle chimera (KChim-W362F+S367T+V377T) reveal that the transition of voltage-gated potassium channels from the open-conducting conformation into the non-conducting inactivated conformation involves the rupture of a key intra-subunit hydrogen bond that tethers the selectivity filter to the pore helix. Breakage of this bond allows the side chains of residues at the external end of the selectivity filter (Tyr447 and Asp449 in K1.3) to rotate outwards, dilating the outer pore and disrupting ion permeation. Binding of the peptide dalazatide (ShK-186) and an antibody-ShK fusion to the external vestibule of K1.3 narrows and stabilizes the selectivity filter in the open-conducting conformation, although K efflux is blocked by the peptide occluding the pore through the interaction of ShK-Lys22 with the backbone carbonyl of K1.3-Tyr447 in the selectivity filter. Electrophysiological studies on ShK and the closely-related peptide HmK show that ShK blocks K1.3 with significantly higher potency, even though molecular dynamics simulations show that ShK is more flexible than HmK. Binding of the anti-K1.3 nanobody A0194009G09 to the turret and residues in the external loops of the voltage-sensing domain enhances the dilation of the outer selectivity filter in an exaggerated inactivated conformation. These studies lay the foundation to further define the mechanism of slow inactivation in K channels and can help guide the development of future K1.3-targeted immuno-therapeutics.